Light-emitting diodes (LEDs) are widely used, inter alia, in lamps or other illuminators, as indicators in electronic devices, and in large-format displays. For example, displays using LEDs are found in outdoor signage and stadium displays.
Inorganic light-emitting diode displays using inorganic micro-LEDs on a display substrate are also known. Micro-LEDs can have an area less than 1 mm square, less than 100 microns square, or less than 50 microns square or have an area small enough that it is not visible to an unaided observer of the display at a designed viewing distance. U.S. Pat. No. 8,722,458 entitled “Optical Systems Fabricated by Printing-Based Assembly” teaches transferring light-emitting, light-sensing, or light-collecting semiconductor elements from a wafer substrate to a system substrate.
In any application requiring many elements, it is important that each element is reliable to ensure good manufacturing yields and performance. Because no manufacturing process is perfect, any large system with many components can have defective elements. In particular, inorganic light-emitting diodes are subject to manufacturing defects that increase the amount of current passing through the LED when provided with power, resulting in an undesirable brightness or power usage, or system control problems, for example.
An undesired increase in current can also overheat the LEDs. One approach to preventing such overheating is to provide each LED with a resistor or group of resistors in series with the LED, as is described in U.S. Pat. No. 9,226,361. U.S. Patent Application Publication No. 2016/0057827 A1 discloses a current-limiting diode in an LED circuit for a lighting system with both forward-biased and reverse-biased LEDs and a polarity switching device. Circuits for sensing current levels and reducing power dissipation are also known, for example as taught in U.S. Patent Application Publication Nos. 2016/0057822 A1 and 2016/0057832 A1, as are voltage-control circuits, for example as described in U.S. Patent Application Publication No. 2016/0056725 A1. However, these approaches either require complex or expensive circuit components in association with each LED or result in diminished light output.
Alternatively, to ensure that large multi-element systems are reliably manufactured and operated, such systems can employ redundant elements. For example, displays are sometimes designed with redundant light emitters. U.S. Pat. No. 5,621,555 describes an LCD with redundant pixel electrodes and thin-film transistors to reduce defects. In another approach, described in U.S. Pat. No. 6,577,367, an extra row or column of pixels is provided to replace any defective row or column. Another approach to improving display yields uses additional, redundant light-emitting elements, for example two light emitters for every desired light emitter in the display. U.S. Pat. No. 8,766,970 discloses a pixel circuit with two sub-pixels and circuitry to determine whether a sub-pixel is to be enabled, for example if another sub-pixel is faulty. Similarly, U.S. Pat. No. 7,012,382 teaches an LED-based lighting system that includes a primary light source and at least one redundant light source. The primary light source is activated by itself and the performance of the light source is measured to determine whether or not to drive the redundant light source. The redundant light source is activated when the performance measurements indicate that a performance characteristic is not being met by the primary light source alone. The first light system can be activated in combination with the redundant light source once the decision is made to activate the redundant light source. U.S. Pat. No. 8,791,474 discloses redundant pairs of micro LED devices driven by a common transistor. WO 2014/149864 describes separately controlled micro-LED devices. However, the use of redundant emitters can be expensive and generally does not address problems with LEDs that conduct too much current.
There is a need, therefore, for LED circuits that can control or avoid problems resulting from LEDs in a display, lamp, or other light-emitting device that undesirably conduct too much current or do not emit the desired amount or color of light or both.